A switching power supply unit controls electric power that provides to a load by turning on and off a switch (e.g. power transistor) connected in series or parallel with the load. Such switching power supply unit uses a smoothing coil and an output condenser in order to stabilize a load current.
The duty factor of the switch controlling the supply of electric power to the load is changed by, for example, regulating the period (or pulse width) of the on-state pulses while fixing the frequency of the pulses.
Switching power supply units are in general more efficient than series power supply units. However, the efficiency of a switching power supply unit depends on the magnitude of the output current. The efficiency goes down with the output current for a small or light load. The lowering of the efficiency is mainly due to the power loss accompanying the switching operation of the power supply unit. The loss is called switching loss.
Therefore, in order to reduce the switching loss of a switching power supply unit under a light load, the unit is harnessed by a burst mode control or a skip mode control.
In the burst mode control, when the output current is detected to be sufficiently small, the power is cut off over a period while an output condenser can substantially sustain a predetermined output voltage, as taught in Japanese Patent Early Publication H-06-303766. That is, on-off switching periods in which power bursts and off-periods alternate. During the off-periods, no switching loss takes place, so that the efficiency is improved accordingly.
In a skip-mode control, a window having a fixed voltage width is applied to the output voltage by a window comparator. In this case, the power supply unit is switched with pulses having a fixed duty factor while raising the output voltage from the lower limit to the upper limit of the window. However, the output voltage is left freely falling from the upper limit to the lower limit. The efficiency is improved while the output voltage is falling from the upper limit to the lower limit, since no switching loss is involved then.
However, under a burst-mode control, burst noises are generated in such conventional switching power supply unit as the unit undergoes a transition from an on-off period to an off-period due to the fact that the switching frequency changes discontinuously in the transition. The burst noise gives adverse effects to the surrounding electronic devices, and can result in mutual interference among them.
A switching power supply unit also results in bursts of output power in a skip-mode control because on-off switching is performed only when the output voltage is raised from the lower limit to the upper limit. Besides, large ripples result in the varying output voltage. Such large ripples in the output voltage are unfavorable for the load.